1. Field of the Invention
The present invention relates to a polarisation separation element that separates incident unpolarised or partially polarised light into two angularly separated output beams having different polarisation states. The invention also relates to a polarisation conversion system that converts light that is unpolarised or partially polarized to light that is substantially completely polarised. The invention also relates to an optical element that comprises two lens arrays disposed on opposite faces of a substrate. The invention also relates to a projection display system incorporating such a polarisation-conversion system and possibly such an optical element.
2. Description of the Related Art
Many optical systems require that they are illuminated by light; that is substantially completely polarised. Where such a device is operated with light that is unpolarised, or that is partially polarised, it is necessary for the light to be completely polarisedxe2x80x94that is, converted to a single polarisation statexe2x80x94before it is incident upon the optical system.
One way of converting unpolarised light to completely polarised light is the well-known linear polariser. An idealised linear polariser transmits light that is linearly polarised in one direction without loss and completely absorbs light that is linearly polarised in an orthogonal direction, so that unpolarised light incident on the polariser is converted into light that is completely linearly polarised. While such a linear polariser is a straightforward means for producing linearly polarised light, it has the disadvantage of having a low efficiency. An ideal linear polariser, in which there is no loss of the polarisation component that is intended to be transmitted owing to absorption within the polariser and/or reflection at the surfaces of the polariser, has an efficiency of only 50%, and the efficiency of a practical linear polariser is generally within the range 40-45%.
Another known means for converting unpolarised light to polarised light is a polarisation conversion system. In a polarisation conversion system, incident light that is already polarised in a desired polarisation state is transmitted unchanged. Light that is polarised in a polarisation state orthogonal to the desired polarisation state is converted to light of the desired polarisation state, rather than being blocked as happens if a conventional linear polarise is used.
A polarisation conversion system consists essentially of a polarisation splitting element (PSE) that splits incident unpolarised or partially polarised light, so that light of one polarisation state is emitted from the PSE spatially or angularly separated from light having an orthogonal polarisation state. A polarisation conversion system also comprises a polarisation conversion element for changing the polarisation state of one of the components emitted by the PSE into the orthogonal polarisation state.
Many polarisation separation elements are known. As one example, FIG. 19 shows an embodiment of the well-known Wollaston prism in which two birefringent wedges W1,W2 are joined to form a composite block, with the hypotenuse faces of the two prisms adjacent to one another. In this embodiment of the Wollaston prism, described in EP-A-0 993 323, the two wedges W1,W2 are embodied as liquid crystal layers having varying thickness. The direction of the optic axis in each wedge rotates through 90xc2x0 across the thickness of the wedge, with the optic axis of the two wedges being perpendicular to one another at the interface between the two wedges.
U.S. Pat. No. 5,978,136 discloses a conventional PCOS, which is illustrated in FIG. 21(a) of the accompanying drawings. This polarisation conversion system comprises two microlens arrays 5 and 6. The elements of the first microlens array 5 image to corresponding elements of the second microlens array 6. A set of polarising beam splitter cubes 2 that contains polarising separation films 2a then spatially separates P and S components of the light so that only the P or only the S component is incident upon a set of retarder stripes 3. The retarder stripes are configured to be substantially half wave plates, such that light incident on a retarder stripe is converted to its substantially orthogonal state. Light leaving the polarising conversion system is now substantially polarised. The polarising beam splitter cubes further contain reflecting films 2b that reflect the other polarisation component so that it leaves the PCOS in a direction that is substantially parallel to the direction in which light leaves the retarder stripes 3. An opaque mask 9 is disposed between the second microlens array 6 and the polarising beam splitter array 2 to reduce cross-talk. The minimum volume of the PCOS is constrained by the tolerances of the half-wave plates.
The set of polarising beam splitter cubes 2 of the PCOS of FIG. 21(a) may be obtained by obliquely cutting a stack of PBS plates 49, as shown in FIG. 21(b). A suitable cutting cross-section is indicated by reference 50 in FIG. 21(b).
Ogiwara et al describe in xe2x80x9cPS Polarisation Converting Device for LC Projector Using Holographic Polymer-Dispersed LC Filmsxe2x80x9d, SID 1999, a further conventional PCOS. This device it shown in FIG. 20 and comprises two microlens arrays 5, 6, two polymer dispersed liquid crystal (PDLC) gratings 2, 4 and a set of half wave retarder elements 3. Polarisation splitting is achieved by the PDLC grating 2 that diffracts substantially only light of one linear polarisation (P) and transmits light having the orthogonal linear polarisation (S) without significant diffraction.
The half wave retardation plates 3 are mounted on the second grating 4 and are arranged in the path of p-polarised light emitted by the grating 2. When p-linearly polarised light passes through one of the half wave plates 3, it will be converted to s-linearly polarised light.
The half wave retardation plates 3 are arranged so that the s-linearly polarised light emitted by the grating 2 does not pass through the half wave retardation plates 3. The s-polarised light emitted by the grating 2 is therefore not affected by the half wave plates 3. After passing through the array of half wave plates, the light is therefore completely s-polarised.
In use, the polarisation conversion system is illuminated by collimated light produced by a lamp 7 and a parabolic mirror 8, and incident light is focused by the first microlens array 5. The second microlens array 6 has a similar focal length and pitch to the first microlens array 6. The first and second microlens arrays are separated by approximately their focal length.
This PCOS again has the disadvantage that the minimum volume is constrained by the tolerances of the half-wave retarder elements. A further disadvantage is that this system uses two polarisation splitting elements to reduce dispersion, and this increases the cost and complexity of the PCOS.
The dimensions of the polarisation conversion system shown in FIG. 21(a) are typically of the order 50 mmxc3x9750 mmxc3x9770 mm. When the polarisation conversion optical system (PCOS) is used with a projector, it significantly increases the overall volume of the projector. The volume of the PCOS of FIG. 21(a) can only be reduced if the focal length of the microlens arrays is reduced, and this requires a corresponding reduction in the pitch of the microlenses, the half way plates and the polarisation splitting cubes. The microlens arrays used in a conventional PCOS of the type shown in FIG. 21(a) would typically have a pitch p of 6 mm, and it would be desirable to reduce this to under 1 mm with a corresponding reduction in optical system throw. It is, however, difficult to do this in the case of a PCOS that incorporates conventional half-wave retarder elements and conventional polarisation splitting cubes, since it becomes difficult to align the elements with one another with the required tolerance. Fabrication and assembly of the PCOS thus become much more difficult. Accordingly, with the elements used in current polarisation conversion systems of the type shown in FIG. 21(a), the physical size of the elements used places a restriction on the minimum volume of the PCOS.
EP 0 887 667 and GB 2 326 729 disclose a method of fabricating a high precision patterned retarder element and the application of such an element to a polarisation conversion optical system comprising an array of beamsplitters.
FIG. 17 shows a further conventional polarisation conversion-system proposed by Minolta. In this PCOS the polarisation separation element 2 is a diffractive optical element (DOE) polarisation splitter. In the device shown in FIG. 17, light in which the plane of polarisation is in the plane of the paper is not deflected, as shown by the solid ray paths. Light polarised in a direction out of the plane of the paper is deflected, as shown by the ray paths in broken lines. The device also comprises a first microlens array 5 for focusing light emitted by the polarisation separation element 2, a conventional large-size array of half wave retarder elements 3, and a second microlens array 6. The device is illuminated by light from a lamp 7 that has been collimated by a parabolic mirror 8 and passed through a UV-IR filter 9xe2x80x2.
The prior art PCOS of FIG. 17 has the disadvantage that it uses a diffractive element as the polarisation separation element 2. Because this is a diffractive element it will suffer from high chromatic dispersion, and will also suffer from polarisation mixing owing to the overlapping of multiple diffraction orders. The high chromatic dispersion of the polarisation separation element will also mean that the efficiency of the PCOS will be low.
FIG. 18 illustrates a further prior art polarisation conversion system. This PCOS is described in U.S. Pat. No. 5,900,977 and in WO97/01779, and consists of three elements that are shown separated in FIG. 18 for clarity.
The first component 10 of the PCOS of FIG. 18 splits unpolarised or partially polarised light into two components propagating in different directions and having orthogonal linear polarisations. The second component 11 is a polarisation-rotating element that rotates the plane of polarisation of light. The rotation of the plane of polarisation produced by the second component 11 is strongly dependent on the angle of incidence of light. Light incident on the component 11 in the normal direction, such as the beam b1, will have its plane of polarisation rotated by 90xc2x0. Light that is incident on the element 11 in a non-normal direction, such as the beams b2, will have their plane of polarisation unchanged.
The third component 12 bends the beams of light, so as to produce a substantially collimated output beam. The first and third, components 10, 12 consist of alternating areas of birefringent material and optically isotropic material.
The polarisation conversion system of FIG. 18 suffers from a low acceptance angle. For example, the acceptance angle of a typical projection system may be of order 5 degrees, whereas for this element a lower acceptance angle for high convergence efficiency may be expected. Such an element may be suitable for use with a laser such as in a CD player.
U.S. Pat. No. 5,440,424 discloses a sheet polarisation conversion system that contains a polarisation-separating component, a polarisation-rotating component and a combining component. This polarisation conversion system also has a low acceptance angle.
EP-A-0 753 780 discloses a polarisation separation element that comprises a liquid crystal layer sandwiched between two substrates. One of the substrate has a serrated surface structure, so that the thickness of the liquid crystal layer is not constant. Unpolarised light incident on the polarisation separation element is split into two different polarisation components at the interface between the serrated substrate and the liquid crystal layer, and the two polarisation components leave the polarisation separation element travelling in different directions.
In the polarisation separation element disclosed in EP-A-0 753 780 one of the polarisation components passes through the polarisation separation element without deviation, nominally for all wavelengths of visible light. Light must therefore be incident on the polarisation separation element at non-normal incidence to prevent significant loss of light. This means that an optical projector using the polarisation separation element of EP-A-0 753 780 is required to use a tilted lamp in order to prevent significant loss of light.
A first aspect of the present invention provides a polarisation separation element comprising: a first array of prisms, each prism having a wedge-shaped cross-section; and a second array of prisms, each having a wedge-shaped cross section; wherein each prism of the first array is disposed with an inclined face disposed adjacent an inclined face of a corresponding prism of the second array; wherein each prism of at least one of the arrays of prisms is a birefringent prism; and wherein the polarisation separation element is arranged to deviate light having the first polarisation and to deviate light hating the second polarisation.
A polarisation separation element of the present invention deviates both the first polarisation component and the second polarisation component. That is, the direction in which the first polarisation component is output from the polarisation separation element and the direction in which the second polarisation component is output from the polarisation separation element are both different from the direction of the incident light. When a polarisation conversion system having a polarisation separation element of the present invention is incorporated in a projection display system, the use of a non-tilted lamp geometry does not lead to additional loss of light assuming the correct materials and prism geometries are used.
Each prism of the first array of prisms may be a birefringent prism and each prism of the second array of prisms may be a birefringent prism. Each prism of the first array may be arranged with its optic axis perpendicular to the optic axis of the corresponding prism of the second array.
Each prism of the first array of prisms may be an optically isotropic prism and each prism of the second array of prisms may be birefringent prism.
The ordinary refractive index no of a prism of the second array, the extraordinary refractive index ne of a prism of the second array and the refractive index n of a prism of the first array may be chosen such that:
no less than n less than ne
The array of birefringent prisms, or one of the arrays of birefringent prisms if there are more than one array of birefringent prisms, may comprise a liquid crystal material.
The polarisation separation element may comprise spacers for determining the thickness of the liquid crystal layer. Each spacer element may be integral with a respective one of the prisms of the first array.
The array of birefringent prisms, or one of the arrays of birefringent prisms if there are more than one array of birefringent prisms, may alternatively comprise a reactive mesogen, or may comprise a polymer-stabilised liquid crystal material.
The polarisation separation element may further comprise a third array of prisms, each having a wedge-shaped cross-section; and a fourth array of prisms, each having a wedge-shaped cross-section; and each prism of the third array may be disposed with an inclined face adjacent an inclined face of a corresponding prism of the fourth array; and each prism of the third array may be a birefringent prism.
The direction of the optic axis of a prism of the second array may vary over the thickness of the prism.
The direction of the optic axis of a prism of the second array may vary by substantially 90xc2x0 over the thickness of the prism, the optic axis being substantially perpendicular to the direction of incident light over the thickness of the prism. The direction of the optical axis of a prism of the second array at the face of the prism disposed closer to the third array of prisms may be perpendicular to the optic axis of the prisms of the third array. The second array of prisms may comprise a liquid crystal layer.
A second aspect of the present invention provides a polarisation conversion element comprising: a first lens array for converging incident collimated light; a polarisation separation element for directing light having a first polarisation in a first direction and for directing light having a second polarisation different from the first polarisation in a second direction different from the first direction: and one or more polarisation conversion elements for converting light having the first and second polarisations to light having a substantially common output polarisation; wherein the polarisation separation element is a polarisation separation element as defined above.
The output polarisation may be the second polarisation.
The array of polarisation conversion elements may be disposed substantially in the focal plane of the first lens array.
The first lens array may be disposed between the polarisation separation element and the polarisation conversion element. Alternatively, the first lens array may be disposed before the polarisation separation element.
The polarisation conversion system may further comprise a second lens array for collimating the output of the polarisation conversion element. The first lens array and the second lens array may have a common substrate. The second lens array may be adjacent and behind the polarisation conversion element.
The polarisation conversion element may be disposed directly on the second microlens array. This prevents the polarisation conversion element becoming mis-aligned from the second microlens array.
The polarisation conversion element may be disposed after and may be optically coupled to the second lens array.
The output from the polarisation separation element may be a first beam of linearly polarised light having a first plane of polarisation and a second beam of linearly polarised light having a second plane of polarisation different from the first plane of the polarisation, and the or each polarisation conversion element may be a polarisation rotation element.
The plane of polarisation of the first beam may be at substantially 90xc2x0 to the plane of polarisation of the second beam.
The one or more polarisation conversion elements may comprise a retarder array having a plurality of first regions alternating with a plurality of second regions, the first and second regions being arranged to receive light of the first and second polarisations, respectively. The first and second regions may have first and second sizes which are matched to the cross-sectional sizes of light beams of the first and second polarisations, respectively, and which are different from each other.
A third aspect of the present invention provides a projection display system comprising a source of unpolarised or partially polarized light, a polarisation conversion system as defined above, and a projection lens.
A fourth aspect of the present invention provides an optical element comprising: a substrate; a first lens array disposed on one surface of the substrate; and a second lens array disposed on an opposing surface of the substrate, each lens of the second lens array being optically associated with a lens of the first lens array.
The first lens array and the second lens array may be integral with the substrate.
An optical element according to this aspect of the invention is suitable for use in a polarisation conversion system of the second aspect of the invention. By disposing both lens arrays on a common substrate the accuracy with which a lens of one array can be aligned with a lens of the other array can be increased, and this enables the pitch of the lens arrays to be reduced. Reducing the pitch of the lens arrays enables their focal lengths to be reduced, so reducing the distance between the two lens arrays and thereby reducing the volume of a polarisation conversion system incorporating the lens arrays.
The pitch of the first lens array may be substantially equal to the pitch of the second lens array. The pitch of the first lens array and the pitch of the second lens array may each be less than 2 mm.
The width W of the optical element and the thickness T of the optical element may satisfy the relationship W/T greater than 3.